From what I understand, illegals come here to work and send the money they make back home. I'm no economist, but doesn't that reduce the value of the dollar?
Mostly I asked a skewed question to screw with the OP.
But, you have two issues. Does sending money from the USA to Mexico devalue the dollar. No, it increase the value because you have more people using dollar. Of course, that really might be overstating it a touch since you could argue the demand for the dollar is there regardless.
Now, I think what you really mean is: doesn't this hurt our economy? I personally believe it's a net positive for the US economy as it adds to our productivity and our population, the two most fundamental forces behind growth. And that isn't to say there aren't dislocations that can be painful for people who "lose their job" to cheaper labor. (Though they were going to lose them anyway in a global economy).
That said, it's a complex issue where lots of people disagree with me.